The Ethernet Passive Optical Network (EPON) is an access technology obtained by combining a passive optical network technology and Ethernet technology. The EPON system mainly includes an Optical Line Terminal (OLT), Optical Distribution Network (ODN) and Optical Network Unit (ONU). The OLT connects with one or more ODNs which are passive optical splitters and used for transmitting downlink data from the OLT to a plurality of ONUs via an optical distributor and transmitting uplink data from the ONUs to the OLT via convergence. Wherein, the data transmission is in a passive optical fiber transmission mode.
At present, an Ethernet Passive Coaxial-cable Network (EPCN) system is developed with reference to the network structure of the EPON system. The EPCN system takes the Ethernet as a carrier, adopts point-to-multipoint structure and a passive Coaxial-cable transmission mode, where uplink data streams are transmitted in a burst Ethernet packets. FIG. 1 is a schematic diagram illustrating structure of the EPCN system. As illustrated in FIG. 1, the EPCN system mainly includes a Coaxial-cable Line Terminal (CLT), a brancher/distributor, and Coaxial-Cable Network Units (CNUs). The EPCN connects with various Ethernet devices via the CLT and connects with user equipment via the CNUs. The EPCN system may be applied to various service environments. At present, the most commonly application of the EPCN system is to provide Ethernet Broad Band accessing service to home users.
The EPCN system is generally used as a semi-duplex system. In such a case, transmission mediums shared by an uplink and downlink are occupied by the downlink from the CLT to each CNU and the uplink from each CNU to the CLT in time-sharing. That is to say, in the downlink transmission period, the CLT transmits downlink data to each CNU, while each CNU does not transmit any information to the CLT, and in the uplink transmission period, each CNU transmits uplink data to the CLT, while the CLT does not transmit any information to each CNU.
In the EPCN system, in order to guarantee normal communication between the CLT and each CNU, a clock of each CNU must be kept synchronous with that of the CLT. However, the ability of each CNU to keep the clock is limited. The CNU generally can guarantee accuracy of its clock in a period from 10 ms to several hundreds ms. If each CNU can not receive clock information from the CLT within the period that the clock synchronization can be kept, there will be an error in the clock kept by the CNU and the clock of each CNU and that of the CLT will not be synchronous, resulting in that each CNU can not perform normal communication with the CLT and Quality of Services (QoS) greatly reduces.